1. Field of the Invention
The present invention relates to a method of and apparatus for imaging within a sample space an object containing quadrupolar nuclei. The object may, for instance, be a human or animal body or a part thereof; the image may be a simple spin-density image or may be a temperature, pressure or electric field distribution image. For convenience the imaging referred to above will be called NQR imaging. The-present invention also relates to a method of and apparatus for NQR testing an object, and to a method of and apparatus for detecting the presence of a particular substance containing a given species of quadrupolar nucleus.
2. Description of the Related Art
Methods for producing imaqes from the resonances of spin-1/2 nuclei which have a magnetic moment but no electric quadrupole moment have been extensively developed and described in various books (e.g. P. Mansfield and P. G. Morris, "NMR Imaging in Biomedicine", 1982, Academic Press). Such methods may use linear magnetic field gradients superimposed on a strong, uniform magnetic field. The resonance frequencies of the spin-1/2 nuclei are linearly dependent on the total magnetic field; this is made to vary linearly with distance across the sample, and the distribution of the substance concerned is derived from the frequencies in the signals produced. Such techniques are commonly called NMR imaging.
Methods for imaging using the resonances of quadrupolar nuclei (which have I.gtoreq.1) would be expected to have several advantages. Firstly, the quadrupolar resonances can be detected without using the strong, uniform magnetic field which is needed to make the magnetic resonances of spin-1/2 nuclei conveniently detectable. Hence the relatively large, expensive and sample-size-limiting magnet structures which are necessary for NMR imaging will not be needed. Avoiding the need for this strong magnetic field also avoids the substantial complications and possible distortions caused by non-uniformity in the field and magnetic interference produced in practical NMR equipment. Secondly, the quadrupolar resonances are more definitely or characteristically associated with specific chemical environments, and it is therefore easier to distinguish results which are due to a particular substance from effects which are due to other substances present in the sample. Methods in which quadrupolar resonances might be observed directly rather than through their interactions with magnetic resonances would be highly suitable for medical use because they do not require patients to be exposed to rapidly changing strong magnetic fields.
Possible disadvantages or limitations on NQR imaging are that the nuclei may be less abundant, may have nuclear quadrupole resonance frequencies lower than the magnetic resonance frequencies of the protons commonly used in NMR imaging, and may have smaller gyromagnetic ratios. This can create a sensitivity problem which has two aspects. Firstly, it may be difficult to achieve a degree of excitation of the quadrupolar nuclei comparable with the excitation of the spin-1/2 nuclei which is commonly used in NMR equipment. The radiofrequency power used in medical applications must be limited to avoid undue heating and damage to living tissue. Secondly, the response signals will be weak and of low signal-to-noise ratio, needing sophisticated data processing for their detection.
To our knowledge methods for imaging using quadrupolar resonances have as yet had very little development. Rommel et al (J. Magnetic Resonance 91, 630-636, 1991) reported theoretical reasoning with a conclusion that there would be great practical difficulties in any attempt to use such methods to derive any image from the interaction of a magnetic field gradient with quadrupolar nuclei, considering in particular half-integral spin nuclei of I.gtoreq.3/2. As an alternative they reported a method in which the strength of a radiofrequency excitation was varied across a sample in zero magnetic field.
Matsui et al. (J. Magnetic Resonance 88, 186-191, 1990) reported measurements on .sup.35 Cl nuclei in sodium chlorate in a linear magnetic field, in which an image was derived from the effects of the field on the spectral shape and width of the resonance. It appears that this may be a special case; the .sup.35 Cl nuclei have I=3/2 and their asymmetry parameter in sodium chlorate is zero. Matsui et al. state that "it is practically impossible" for the magnetic field to shift the resonance frequency. This may be incorrect, at least in relation to nuclei other than .sup.35 Cl in substances other than sodium chlorate.